This invention relates to a plasma device for processing a sample or samples by the use of plasma.
As a plasma device of the type described, a dry etching device has been used which comprises a pair of flat electrodes located in parallel to each other in a hollow space. During etching, an a.c. voltage of a high frequency more than several tens of kiloherz is usually supplied from an a.c. voltage source to the electrode pair to produce plasma in the hollow space between the electrode pair. Such a dry etching device is indispensable to a very large scale integration technique because it is possible for the dry etching device to delineate five patterns on a sample, such as a semiconductor wafer. It is mentioned here that the dry etching device is loaded with the sample selectively covered with a resist layer having resist patterns and lends itself to transcribe the resist patterns to the sample so as to leave the five patterns on the sample.
Such a resist layer is poor in thermal stability and is, therefore, susceptible to thermal damage by bombardment of electrons and the like. When the resist layer is subjected to the thermal damage, precise transcription of the resist patterns becomes difficult. As known in the art, it is necessary to augment high frequency electric power of the a.c. voltage source in order to raise an etch rate in the dry etching device. With augmentation of the high frequency electric power, the thermal damage of the resist layer becomes more serious.
It has been pointed out that a sample support, such as one of the electrode pair, may be cooled during etching to reduce a temperature of the sample and to alleviate the thermal damage of the resist layer. To this end, the sample should fixedly or firmly be brought into contact with the sample support. However, it is not always easy to fixedly bring the sample, for example, the wafer into contact with the sample support because the wafer is thin and very light. In addition, conduction of heat is not good enough in an atmosphere of a low pressure gas in the hollow space. The wafer is, therefore, not preferably cooled in the hollow space kept at a low pressure.
In order to fully cool the wafer, it is necessary to snugly attach the wafer to the sample support. For this purpose, the wafer has usually been pressed to the sample support by the use of clamps or weights. An arrangement of the clamps or weights results in partial disturbance of plasma during etching. The wafer is not uniformly processed by the disturbance of the plasma. In addition, provision should be made of a number of the clamps or weights when a plurality of the samples are concurrently processed. Thus, use of the clamps or weights is not suitable for processing a lot of the samples.
In another way, grease has been coated on the rear surface of each sample to attach each sample to the sample support. Use of the grease, however, gives rise to contamination of the sample because the grease is spread over a boundary of the sample.
Alternatively, a dry etching device disclosed in Japanese Pre-patent Publication or Published Unexamined Patent Application No. Syo 55-90228, namely, 90228/80 fixedly attaches a sample to a sample support by the use of electrostatic attraction, as will later be described with reference to one of a few figures of the accompanying drawing. More particularly, the sample support comprises a particular one of electrode pair supplied with a high frequency a.c. voltage and a support member on the particular electrode. The particular electrode may be called a high frequency electrode. The support member comprises a dielectric layer brought into contact with the sample. The particular electrode is supplied with a d.c. voltage in superposition on the high frequency a.c. voltage. In this structure, the electrostatic attraction results from interactions of the d.c. voltage and a self-bias voltage occurring during production of the plasma. As a result, the sample is attracted to the particular electrode by the electrostatic attraction and is desirably cooled by a coolant flowing through the particular electrode. Therefore, the sample is fixedly held on the electrode without any thermal damage.
With the above-mentioned dry etching device, the d.c. voltage is superposed on the high frequency a.c. voltage on the particular or high frequency electrode. This means that the plasma causes electrical charges to flow from the particular electrode into a d.c. voltage source when the high frequency electrode is unexpectedly exposed to the plasma. At times, the electrical charges bring about a reduction or extinction of the electrostatic attraction and a destruction of the d.c. voltage source. Exposure of the high frequency electrode may give rise to variation of a plasma potential and to unusual discharge. Thus, etching is adversely affected by the variation of the plasma potential and the unusual discharge. In addition, the etching device should be handled with a great care because a coolant, such as water, is not isolated from the d.c. voltage source.
At any rate, it is difficult for the conventional plasma devices to concurrently process a plurality of samples at a high etch rate.